This invention relates to a heat sink which is formed from a metal sintered compact produced by sintering an injection molded body prepared from metal powders. This invention also relates to a process for producing a metal sintered compact such as a heat sink.
A semiconductor chip used for CPU or the like generates heat in its operation, and therefore, a heat sink or a heat dissipater is attached to such semiconductor chip for facilitating the heat dissipation or radiation.
Heat sinks are generally produced from tungsten (W)-based metal materials since heat sinks must have a small coefficient of expansion to avoid the cracking of the semiconductor chip by the temperature difference generated between the heat sink and the semiconductor chip formed of silicon (Si).
Recently, use of a W-Cu alloy is also contemplated as a material for heat sinks. The W-Cu alloy is an alloy wherein copper (Cu) having a good thermal conductivity is added to tungsten to thereby improve the heat dissipation properties.
Conventionally, metal products such as heat sink have been produced by a casting. However, it has been difficult to produce a small precision metal product of intricate shape by such casting. Therefore, such casting method is inappropriate for producing a heat sink, because a heat sink inevitably has an intricate geometry for facilitating heat radiation.
In view of such situation, metal injection molding (MIM) has been proposed for use in place of the casting. In MIM, the metal powder is mixed with an organic binder, and then the mixture is injection molded to produce a green body. Such a green body is then sintered in a sintering furnace to produce a sintered compact.
A metal sintered compact of a W-Cu alloy, however, suffers from the following drawbacks.
(1) The W-Cu sintered compact exhibits a low sintered density, namely, it has a high porosity. Therefore, the sintered compact has low strength and therefore it is brittle, so that the sintered compact is likely to break by an impact and the like.
(2) When the surface of the heat sink which is to be in contact with the semiconductor chip is plated with Ag or an alloy thereof, the adhesion of the plated layer with respect to the sintered compact is relatively low, thus leading lifting and peeling off of the plated layer.
Such a relatively low adhesion of the plated layer is believed to be resulted from the properties of the W-Cu alloy. Further, in addition thereto, it is also believed that the plating solution of considerable volume remaining in the pores of the sintered compact of high porosity as described in (1) above affects such relatively low adhesion of the plated layer.
(3) The sintering process requires use of hydrogen gas or an inert gas containing hydrogen gas for the sintering atmosphere. The use of such gas renders the process rather dangerous.
As described above, the metal sintered compact of a W-Cu alloy has been insufficient in reliability due to the reasons stated in (1) and (2) above, and its production was neither easy nor safe due to the reason stated in (3) above.
In view of the situation as described above, a main object of the present invention is to provide a sintered compact which has a reliable quality and which is easy and safe to produce.
Another object of the present invention is to provide a production process of such sintered compact.
In order to achieve these objects, one aspect of the present invention is directed to a sintered compact for use in a heat dissipater. The sintered compact formed from metal powders and having a metal composition comprising at least one material selected from the group comprising tungsten and molybdenum, and 2 to 50% by weight of silver.
According to the sintered compact, when the sintered compact is used for a heat sink for a semiconductor chip for instance, it is possible to prevent cracks effectively. Further, dissipation of heat is improved due to silver added thereto, and the adhesion of the plating layer is also improved due to use of silver or silver-alloy plating layer.
In the present invention, it is preferred that the sintered compact further comprises not more than 10% by weight of a transition metal. In this case, it is preferred that the transition metal is at least one metal selected from the group comprising Fe, Ni and Co.
By constructing the sintered compact in this way, it is possible to lower the porosity of the obtained metal sintered compact as well as to improve the adhesion of the plated layer. Further, it is also possible to prevent alternation (tarnishing) of the plated layer as well as to lower the sintering temperature.
Further, in the present invention, it is also preferred that the sintered compact further comprises phosphorus. This is effective in lowering the sintering temperature.
In the present invention, a part of the silver content in the sintered compact which is not more than 50% may be substituted with copper. Such a substitution can reduce an amount of use of silver which is an expensive material without changing the characteristics, thereby enabling to reduce manufacturing costs.
Furthermore, it is preferred that the sintered compact according to the present invention has a thermal conductivity of at least 140 Wxc2x7mxe2x88x921xc2x7Kxe2x88x921 at a temperature of 20xc2x0 C. In this way, it is possible to exhibit excellent heat dissipation ability when it is formed into a heat sink for a semiconductor chip.
Moreover, it is also preferred that the sintered compact according to the present invention has a thermal expansion coefficient of 12xc3x9710xe2x88x926 Kxe2x88x921 or less. In this way, it is possible, when it is used as a heat sink for a semiconductor chip, to prevent that cracks will occur in the semiconductor chip.
In the present invention, it is also preferred that the sintered compact has a porosity of 10% or less. By constructing the sintered compact in this way, it is possible to prevent the mechanical strength thereof from being lowered, so that such sintered compact is difficult to be damaged even if it impact is applied thereto. Therefore, the reliability of the heat sink is improved. Further, the adhesion of plating is also improved.
The sintered compact as described above is preferably used as a heat sink for cooling a semiconductor chip.
In the present invention, it is also preferred that the sintered compact has a contacting surface adapted to contact with a heat generating component, and a coating layer is formed on the contacting surface of the sintered compact. Such a coating layer can give various functions such as a bonding function to the contacting surface.
Preferably, the coating layer is provided for the purpose of providing bonding between the sintered compact and the heat-generating component or assisting such bonding therebetween. In this case, the coating layer is preferably formed into a plated layer.
In this case, the pleated layer is preferably formed from silver or silver alloy. In this way, it is possible to obtain a sintered compact having a desired bonding characteristic and a good thermal conductivity.
Another aspect of the present invention is directed to a method of producing a sintered compact. This method comprises the steps of: [1] producing an injection molded green body according to a metal injection molding; [2] debinding the green body to produce a brown body; and [3] sintering the brown body at a high temperature to produce a sintered compact having a metal composition comprising at least one metal selected from the group comprising tungsten and molybdenum; and 2 to 50% by weight of silver.
According to the method described above, it is possible to easily produce a heat sink for a semiconductor chip which can prevent cracks from being formed in the semiconductor chip and has an excellent heat dissipation ability.
In this method, it is preferred that the sintered compact further comprises not more than 10% by weight of a transition metal. In this case, it is preferred that the transition metal is at least one metal selected from the group comprising Fe, Ni and Co.
By constructing the sintered compact in this way, it is possible to lower the porosity of the obtained metal sintered compact as well as to improve the adhesion of the plated layer. Further, it is also possible to prevent alternation (tarnishing) of the plated layer as well as to lower the sintering temperature.
In this case, it is particularly preferred that the sintered compact further comprises phosphorus. This is effective in lowering the sintering temperature.
Further, in this method, a part of the silver content in the sintered compact which is not more than 50% may be substituted with copper. Such a substitution can reduce an amount of use of silver which is an expensive material without changing the characteristics, thereby enabling to reduce manufacturing costs.
Furthermore, in this method, it is preferred that the debinding process is carried out at a temperature of from 100 to 750xc2x0 C. for a period of 0.5 to 40 hours. In this way, it is possible to carry out the debinding process more preferably and effectively, thus leading to improvement in the quality of the obtained sintered compact.
Further, in this method, it is also preferred that the sintering process in step (3) is carried out for 10 hours or less. In this way, it is possible to prevent that the productivity is lowered.
Furthermore, in this method, it is also preferred that the sintering process is carried out in a hydrogen-free, non-oxidizing atmosphere. This improves safety during the sintering process.
More over, in this method, it is preferred that the sintered compact produced in step (3) has a contacting surface adapted to be in contact with a heat generating component, and the process further comprises the step of performing a surface treatment to at least the contacting surface of the sintered compact which is adapted to be in contact with the heat-generating component. Such a coating layer can give various functions such as a bonding function to the contacting surface.
In this method, it is preferred that the surface treatment is plating. More preferably, the surface treatment is plating by means of silver or silver-alloy. In this way, it is possible to obtain a sintered compact having a desired bonding characteristic and a good thermal conductivity.
Other objects, structures and effects of the present invention will be apparent from the following description of the invention.